This invention relates to cameras, and more particularly to improvements in the camera, disclosed, for example, in U.S. Pat. application Ser. No. 830,036 filed Sept. 2, 1977, and now U.S. Pat. No. 4,165,930 issued Aug. 28, 1979 by utilizing a diffraction device such as hologram for displaying information in the field of a view finder.
Of the various diffraction devices which exist, the so called volume type diffraction device has a three-dimensional structure as shown in FIG. 1. This diffraction grating structure uses either density or refractive index distribution. The characteristic feature of such a volume type diffraction devices resides in that the diffraction efficiency can be maximized only for a light beam having a particular angle of incidence, provided that proper conditions prevail. In other words, such a diffraction device has its share of strong directional characteristics in terms of diffraction efficiency, as shown in FIG. 2. Here, the diffraction efficiency is shown as varying with the angle of incidence of a monochromatic light illuminating the volume type diffraction device.
A phase volume type hologram is one type of such volume type diffraction devices and has a diffraction grating structure which uses refractive index distribution. This phase volume type hologram is characterized in
that (1) the diffraction efficiency is very large;
that (2) the range of angles for the maximum diffraction efficiency is narrow;
that (3) the hologram can be made transparent; and
that (4) the reproduced image is formed on a hologram dry plate.
The use of this phase volume type hologram in an information display device of a camera or the like as arranged, for example, in the finder system thereof, produces little adverse effect on the viewfield of the finder (the darkening of the viewfield, feeling of annoyance because of the unnecessary presentation of a hologram pattern, etc.). Further, the presentation of information can be controlled because a light beam can reproduce the indicia recorded in the hologram only when the angle of illumination coincides with a certain particular value. By changing the wavelength of illumination light and the angle of illumination it is also made possible to display multiple informations.
In practice, however, the application of such diffraction device in a camera for purposes of displaying information in the viewfield of the finder gives rise to many problems. For example, as has been mentioned above, this kind of diffraction device is generally constructed in an emulsion medium in the diffraction grating form. Since the emulsion is susceptible of oxidation, the diffraction device on the base of such emulsion is apt to lower its diffraction efficiency. And, the loss of diffraction efficiency lowers the brightness, and the sharpness of the reproduced image. Another problem is that as the humidity varies, the emulsion shrinks or swells so that the color tone of the reproduced image is varied extensively. In general, the atmosphere in which the camera is used varies over a wide range. Therefore, some provision must be made for always assuring a clear display of information despite variation of ambient atmosphere, so that the reproduced image is stabilized against the reduction of brightness and sharpness, variation of color tone, and the like. Aside from this, there is still the problem that such a diffraction device restricts the position of the reproduced image within narrow limits itself (for example, in a phase volume type hologram, that position coincides with the hologram dry plate.) Therefore, in cameras, to achieve a clear display of information in the field of view of the finder, it is also necessary to take into account where to dispose the hologram.
An information display apparatus utilizing such a diffraction device may be considered as being divided into two parts, one of which includes the diffraction device and serves as the source of indicia to be displayed, and the other of which is a reproduction system including an illumination light source. For arrangement of the diffraction device for the maximum diffraction efficiency, the relative position of both parts is automatically fixed. Hence, the space which the apparatus is intended to occupy is large in comparison with the interior of the finder system. On the other hand, modern cameras are extremely compact and leave little space for appropriate incorporation of an additional new apparatus. The above-mentioned information display apparatus could not be incorporated in the camera without sacrificing compactness. Further, the provision of such an apparatus in a camera with minimum bulk and size requires great effort in arranging of those of the parts of the camera which surround the apparatus in order that requirements of the indicium source and the illumination source for maximum diffraction efficiency prevail. A further problem is that the indicium source and the illumination source must be assembled under specific conditions. Hence, difficulties are introduced into the working operation, and the necessity of fine adjustment for obtaining the maximum diffraction efficiency, in particular, decelerates the speed of a production run.